One of the fundamental functions of printing presses is an accurate, error-free application of images, especially the superimposition of individual single-color images, which then form a composite multi-color image. For this purpose, the so-called color-to-color registration marks are used, which are applied onto the conveyor belt or onto a sheet carried on such conveyor belt. This characteristic feature is called image plane registration. In order to define the image plane registration, special register marks are made outside the printed image, by which the operator of the printing press can determine and measure deviations from properly positioned printing.
In a more advanced version of this procedure the image plane registration is determined and calculated by sensors and computer control located in the printing press. The sensors scan the register marks on the conveyor belt or on the sheet and, using the scanned position of the register marks, the computer control determines whether the printing process occurs error-free with respect to the image plane registration. Any register discrepancy is eliminated by a closed-loop control system.
For this purpose an actual position of the register marks is compared with a reference position and the difference is then used to correct the image plane registration. U.S. Pat. No. 5,893,658 discloses an apparatus for registering multiple image planes of a single image in an electrographic system including an image-printing receptor drum, an image-printing device to create overlaying single-color images on the image-printing receptor drum, at least one developer station, a measuring device for measuring the rotational position of the receptor drum, a drive mechanism for controlling a motor coupled to the receptor drum by at least one drive belt, and a closed-loop positioning system connected with the measuring device and the drive mechanism, whereby the closed-loop positioning system modulates the angular velocity of the receptor drum to guarantee proper image plane registration. Depending on the transit times of the sheets on the conveyor belt, correction parameters to correct any register discrepancy are used for the current sheet to be printed in a printing module, wherein these parameters relate to a sheet that is scanned by a sensor at the end of the conveyor belt. Therefore, the correction of the image plane registration by the correction parameters occurs in relation to an error determined by a sensor at the end of the conveyor belt.
In reality, the size of the register discrepancy changes, for example, by any change in the circumference of the printing drum, and during the time period, in which the sheet is transferred by the conveyor belt from the printing module, in which it has been printed, to the end of the conveyor belt, where it is scanned by a second sensor. Thus, due to the described effect, the determination and elimination of the register discrepancy is not totally accurate. It is desirable to provide a correction parameter in such a manner that such a correction of any register discrepancy can be performed that is related to a sheet located in the nip of the printing module and not to a sheet that is being scanned by a sensor at the end of the conveyor belt.